1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for transmitting ultrasonic waves into a living body and imaging functional information about the movement of moving targets including blood flow in the living body.
2. Description of the Related Art
Such an ultrasonic imaging apparatus is designed to obtain a tomographic image (a B-mode image) and blood flow information by utilizing pulsed ultrasonic waves and the ultrasonic Doppler effect. The measurement of blood flow velocity by the ultrasonic imaging apparatus is made as follows
That is, an ultrasonic beam is transmitted to blood flow within a living body by an ultrasonic transducer. At this time the ultrasonic beam is scattered from moving blood cells so that its center frequency fc is varied by a frequency fd due to Doppler shift. The receive frequency f is thus given by EQU f=fc+fd
The frequencies fc and fd are related by EQU fd=2v cos.theta. fc/C
where v is the velocity of blood flow, .theta. is an angle made by the ultrasonic beam and the blood vessel and C is the velocity of sound. Therefore, the detection of the Doppler shifted frequency fd will allow the blood flow velocity v to be obtained.
In order to display the blood flow velocity v as a two-dimensional image on the basis of the above principle, the following ultrasonic scanning is made.
A subject under examination is subjected to sector scan or linear scan by pulses of ultrasonic waves from an ultrasonic transducer. In this case, several pulses of ultrasonic waves are transmitted in the same direction of raster and ultrasonic waves reflected from the blood flow within the subject, that is, echoes are received by the same ultrasonic transducer. The ultrasonic echoes are converted to electrical signals, i.e., echo signals. The echo signals are entered into a phase detector where Doppler shifted signals are extracted from the echo signals. At this time, the Doppler shifted signals corresponding to, for example, 256 sampling points which are set along the transmission path of the ultrasonic pulses are extracted. Each of the Doppler shifted signals extracted at a respective one of the sampling point is frequency analyzed by a frequency analyzer and then converted to a television signal by a digital scan converter (DSC) for display on a television monitor. Thereby, a velocity profile of blood flow in one scanning direction is displayed as a two-dimensional image in real time. The scanning is also made in the first through nth raster directions so that a blood flow image (a velocity profile of blood flow) is displayed for each of the rasters.
Incidentally, the detecting capability for low velocity of flow depends on the data length of a Doppler shift signal to be frequency-analyzed. That is, assuming a sampling frequency of the Doppler shift signal to be fr and the number of samples to be n, the data length T of the Doppler shift signal to be frequency-analyzed will be given by EQU T=n/fr (1)
Then, the frequency resolution fd will be given by EQU fd=1/T (2)
Thus, the lower limit fdmin of the measurable blood flow will be given by EQU fdmin=1/T=fr/n (3)
In order to detect low-velocity blood flow as well, therefore, it is necessary only that the sampling frequency fr of the Doppler shift signal be made low or the number n of samples be made large. However, two-dimensional Doppler information is obtained from the following equation. EQU FN.times.n.times.m.times.(1/fr)=1 (4)
where FN is the number of frames, m is the number of scan lines and fr is the repetition rate frequency of ultrasonic transmit pulses. The number FN of frames is related to real time processing of two-dimensional blood flow image and generally lies in the range 8 to 30, thereby providing 8 to 30 images per second.
If, in the case of electronic sector scanning, the number m of scan lines is m=32, the repetition rate frequency fr of ultrasonic pulses is fr=4 KHz and the number n of samples is n=8, then the number FN of frames will be about 16. Also, there is the following relationship between the maximum depth of field-of-view Dmax and the pulse repetition rate frequency fr. EQU Dmax=C/(2.times.fr) (5)
Therefore, a problem arises in that if fr is made high in order to improve the number of frames, the maximum depth of field-of-view cannot be made large. Also, if the number m of scan lines is made low, the scanning density will become coarse, degrading image quality. In summary, then, if the number of frames is improved, other properties are degraded.
To solve the above problems, a sequential alternate scanning method and a constant interval alternate scanning method have been developed. Those scanning methods are disclosed and described in U.S. patent application Ser. No. 228, 590 now U.S. Pat. No. 4,993,417 which is assigned to the same assignee as this application.
With the sequential alternate scanning method, as described in detail in the U.S. patent application, a fixed number of rasters in each of a plurality of scanning areas, i.e., a plurality of scanning blocks are scanned repeatedly and echo data at a predetermined number of sampling points of each raster are extracted block by block, thereby forming a blood flow profile.
According to the sequential alternate scanning method, the effective repetation rate frequency fr' of an ultrasonic transmit beam in the same direction is represented as follows: EQU fr'=fr/D (6) (a specific raster is scanned once each time D rasters are scanned)
The lower limit fdmin of the measurable blood flow can be decreased to 1/D of that of the conventional system described previously in which the transmission of an ultrasonic beam is repeated n times in the same direction and then the transmission of an ultrasonic beam is likewise made n times for the adjacent scan line. With this method, however, since the scanning area is divided into a plurality of blocks for scanning according to the sequence of transmission and reception of ultrasonic waves, there is a great time phase difference between rasters in each of blocks. This produces discontinuity in one frame of image. The more the number of rasters in each block in particular, namely, the number of alternate scans, the further the time phase difference deteriorates.
To decrease the time phase difference the constant interval alternate scanning method has been developed. According to this constant interval alternate scanning method, the scanning of a plurality of rasters is repeated a predetermined number of times with one raster being shifted for each scanning and data are output at constant intervals. The constant interval alternate scanning method will decrease the repetition frequency (the sampling frequency of the Doppler signal) of ultrasonic beams transmitted in the same direction to 1/D as with the sequential alternate scanning method described previously and moreover will uniform the time phase difference in one frame because data can be output at constant intervals. Even with this method, however, because the direction of the previous raster shifts, a residual echo signal in the outside of the depth of field-of-view will enter an image signal for the next raster. The residual echo signal will appear as a phase difference, thus generating residual multi-color noise on a display image. By way of example, into the ultrasonic scan line No. 3 is entered echo signals from the scan lines No. 2 and No. 4 immediately before and behind the scan line No. 3. Where there is a residual echo signal, therefore, an appreciable large phase difference will be generated between scan lines. Resultant residual multi-color noise will become a Doppler signal to degrade image quality.